PHOSRESTIN-I, AN ARRESTIN HOMOLOG THAT UNDERGOES LIGHT-INDUCED PHOSPHORYLATION IN DIPTERAN PHOTORECEPTORS

Two classes of phosphorylated homologs of vertebrate arrestins, design ated phosrestins I (PRI) and phosrestin II (PRII), are expressed in th e photoreceptors of a fruit fly, Drosophila melanogaster. This study p resents evidence that the housefly, Musca domestica, also has a protei n similar to Drosophila PRI. Our conclusion is based on the following evidence. (1) We identified a Musca photoreceptor protein exhibiting a molecular mass (51 kDa) and an isoelectric point (pI = 8.6) similar t o those of Drosophila PRI. This Musca protein, designated Musca PRI, c hanges its pI upon illumination in vivo. (2) Rabbit antibodies raised against Musca PRI, against bovine arrestin, and against a synthetic pe ptide based on the Drosophila PRI sequence stained the Drosophila and Musca PRIs specifically on 1 and 2-dimensional Western immunoblots. (3 ) Both Drosophila and Musca PRIs incorporated P-32-radioactivity from gamma-P-32-ATP in cell-free homogenates of retinas. Partial peptide di gestions of Drosophila and Musca PRIs revealed similarity between thes e proteins. We observed that Drosophila PRI exists in the random prepa ration, but it also exists in other subcellular fractions. Immunocytoc hemistry at the EM level revealed a distribution of both Drosophila an d Musca PRI epitopes in membranous vesicular structures in the cytosol as well as in the rhabdomeric microvillar membranes where the visual pigment, rhodopsin, exists. Such distribution of PRI epitopes suggests that PRI and its light-dependent phosphorylation may function in a sp ace remote from the rhabdomere as well as the immediate milieu of phot oreception.